THE PETROLOGY OF THE THOLEIITES THROUGH MELILITE NEPHELINITES ON GRAN-CANARIA, CANARY-ISLANDS - CRYSTAL FRACTIONATION, ACCUMULATION, AND DEPTHS OF MELTING

We report major and trace element X-ray fluorescence (XRF) data for ma fic volcanics covering the 15-Ma evolution of Gran Canaria, Canary Isl ands. The Miocene (12-15 Ma) and Pliocene-Quaternary (0-6 Ma) mafic vo lcanics on Gran Canaria include picrites, tholeiites, alkali basalts, basanites, nephelinites, and melilite nephelinites. Olivine +/- clinop yroxene are the major fractionating or accumulating phases in the basa lts. Plagioclase, Fe-Ti oxide, and apatite fractionation or accumulati on may play a minor role in the derivation of the most evolved mafic v olcanics. The crystallization of clinopyroxene after olivine and the a bsence of phenocrystic plagioclase in the Miocene tholeiites and in th e Pliocene and Quaternary alkali basalts and basanites with MgO > 6 su ggests that fractionation occurred at moderate pressure, probably with in the upper mantle. The presence of plagioclase phenocrysts and chemi cal evidence for plagioclase fractionation in the Miocene basalts with MgO < 6 and in the Pliocene tholeiites is consistent with cooling and fractionation at shallow depth, probably during storage in lower-crus tal reservoirs. Magma generation at pressures in excess of 3-0-3-5 GPa is suggested by (a) the inferred presence of residual garnet and phlo gopite and (b) comparison of FeO(t) cation mole percentages and the CI PW normative compositions of the mafic volcanics with results from hig h-pressure melting experiments. The Gran Canaria mafic magmas were pro bably formed by decompression melting in an upwelling column of asthen ospheric material, which encountered a mechanical boundary layer at ap proximately 100-km depth.